Semiconductor laser diodes have found applications in a wide variety of information handling systems, because of their compact size and because their technology is compatible with that of the associated circuitry and electro-optical elements. They are presently used in areas such as data communication, optical storage and optical beam printing. Currently, most lasers consist of III/V compounds. Depending on the required laser beam wavelength, AlGaAs and InP system devices have found extensive usage.
Presently, the most commonly used laser structures are those where the mirrors terminating the laser cavity are obtained by cleaving. Normally, a wafer carrying a large number of epitaxially grown laser structures is cleaved into laser bars, the cleaved facets at both sides thereof determining the length of the cavity of the devices on the bar.
The cleaving process is normally started by diamond scratching or scribing the wafer surface to determine the crystallographic planes where cleaving is to take place. The scratch serves as a microcrack from whereby applying a bending moment, a controlled fracture completes the cleaving process. The required torque or shearing force is provided by wedges, "knives" or other small-radius tools.
Numerous processes and devices for laser cleaving have been suggested and pursued in the past. Some of these approaches are described in the following references:
German patent application 1,427,772 describes a process for breaking a scribed semiconductor wafer into individual bars. The wafer, held between foils, is placed on a soft-surface support. Cleaving is achieved by a hard, small-radius roller pressed against and moved across the wafer to provide the required torque.
U.S. Pat. No. 3,396,452 briefly described in the introduction and referenced in FIG. 3, is a conventional cleaving tool. It comprises a convex base member with a curvature compatible with the dimensions of the bars into which the wafer sandwiched between foils is to be broken. A non-uniform bending moment is applied by a complementary concave member pressed against the wafer which rests on the convex surface. The invention disclosed in the U.S. Patent relates to a method and apparatus where a scribed semiconductor wafer is advanced and subjected to pressure between a pair of opposing small-radius rollers of different resiliencies. The softer resiliency roller, being contiguous with the scribed wafer face, causes the wafer to break at the scribe lines.
"Process for Batch Cleaving GaAs and Similar Materials", published in the IBM Technical Disclosure Bulletin, Vol. 23, No. 10, March 1981, pp 4749-4750, provides a summary of the main requirements for controlled, non-damaging cleaving processes. It does not refer to any specific details of the process or the apparatus used.
European Patent Application 88.810 694.5, filed on Oct. 10, 1988, describes a method and apparatus for cleaving a semiconductor wafer. The wafer is sandwiched between two elastic foils: a soft lower foil with an adhesive surface to which the wafer "sticks", and a stronger upper foil. The foils are then fixed and stretched. The wafer, however, prevents the stretching, at least within the section of the lower foil to which it adheres. By passing a small-radius roller under the foils, the wafer is lifted and cleaved. The bars thus obtained still adhere to the now completely stretched lower foil, separated from each other by a distance that is sufficient to avoid damage from neighbouring cleaved mirror surfaces.
The methods and apparatus described in the above references do not meet the requirements of a high-quality batch cleave process which include:
gentle, stress-free fracturing along the desired crystallographic plane,
avoidance of damage to device structures due to external forces applied thereto during cleaving,
minimum handling of both the wafer and the resulting bars,
separation of the bars upon cleaving to prevent mechanical damage of neighbouring facets,
obtaining uncontaminated facets followed by immediate application of a uniform passivation coating, and
applicability in a vacuum system.
As a result, major problems are still observed that affect the life-time and output power characteristics of the cleaved devices.
Recently, substantial progress has been made towards resolving these problems. It has been found that cleaving in a vacuum immediately followed by an in-situ coating with a passivation layer of the contamination-free mirror facets allows the fabrication of highly stable AlGaAs laser diodes. With this method, devices have been obtained with very low degradation rates and high-power output performance.
A similar process has been disclosed in European Patent application 89.810 668.7, filed on Sep. 7, 1989. The embodiment described in the specification is suitable for a single bar or device processing, but is not directly applicable to volume laser device manufacturing. It does not address nor solve the additional problems encountered with batch processing such as the restrictive requirement of separating the bars immediately after cleaving in order to avoid facet damage.
Accordingly, it is a main object of the present invention to provide a method and apparatus for semiconductor wafer cleaving suitable for large scale or batch mode laser diode fabrication.
Another object is to provide a method and apparatus allowing cleaving of laser wafers in a high vacuum environment followed by in-situ passivation of the cleaved facets to avoid facet contamination.
A further object is to provide a method and apparatus for cleaving semiconductor wafers to form individual laser bars, each having cleaved facets, and to immediately separate neighbouring bars from each other, to avoid damage, and to allow selective coating of the facets.